Crude oil is processed through petroleum refineries in order to separate the various hydrocarbon products from each other in the crude. Much of this processing takes place at elevated temperatures which reach as high as 700 F. in the distillation columns.
Raw crude oil contains corrosive elements which cannot be removed in the field. Of primary importance is brine which characteristically makes up from about 0.2 to 2.0 percent of the incoming raw crude. Brine contains chloride salts, primarily the magnesium, calcium and sodium varieties thereof.
The desalting operation removes a significant amount of these impurities. However, small amounts of the chloride salts remain with the desalted crude as it is charged into the distillation unit.
During the elevated temperature processing of the crude charge in the distillation unit, the chloride salts, primariy MgCl.sub.2, are hydrolyzed via the following reaction: ##STR1## HCl is then carried overhead in the fractionation towers. Since it is a highly corrosive acid, the HCl will attack and corrode the metallic surfaces throughout the fractionation unit and the upper regions of the distillation columns. The HCl can also combine with volatile basic materials, such as NH.sub.3, to form corrosive salt deposits on tower internals.
Corrosive destruction of and deposition onto these metallic components require corrective action in the form of costly and time consuming repairs. This is detrimental to the cost effective operation of the entire refinery.
A commonly used method to control the evolution of HCl involves the addition of NaOH. This is generally done by injecting an aqueous solution of NaOH into the desalted crude oil charge line. The function of the NaOH is to react with the readily hydrolyzable salts to form NaCl and the corresponding hydroxide. NaCl is much more resistant to hydrothermal decomposition than, for example, MgCl.sub.2. This results in the generation of less HCl.
NaOH is an effective inhibitor of HCl evolution, but its use is accompanied by various negative factors. The choice of this treatment program to reduce chloride concentrations requires the use of large excesses of NaOH. There are two primary reasons for this. First, NaOH is a strong base which will react with acidic species present in all crude oils to various degrees. These acidic species include carboxylic acids, including naphthenic acids, and H.sub.2 S. The second reason for the requirement of large quantities of NaOH is due to its poor dispersibility in crude oil which renders it functionally less efficient.
Excessive amounts of NaOH cause further problems for the refinery operator. NaOH results in caustic cracking and embrittlement near the feed point and causes increased deposition of excess caustic, Mg (OH).sub.2, CaCO.sub.3, etc. in the crude preheat section. Additionally, the practice, described above, produces an increased concentration of Na.sup.+ in the bottoms products which necessitates further processing or results in the production of a lower grade end product. Furthermore, higher Na.sup.+ content of bottoms products causes an increased rate of poisoning of downstream catalytic units. These negative factors are significant economic disincentives militating against the use of NaOH. If NaOH were not used, however, increased corrosion and deposition would result.